The invention relates to a plasma burner which has contact protection means for protecting the operator from impermissible high voltage and the burner head from destruction.
The device is preferably used for burners with which have a transmitted electric arc and in which the nozzle, nozzle cap or other metallic nozzle attaching elements carry a voltage in relation to the workpiece.
Known plasma burners, especially manual plasma burners, are equipped with various devices for the protection of the operator but which do not provide complete protection. It is known to provide the nozzle with a ceramic cap or to coat the nozzle with a layer of ceramic or enamel. The sprays or metal vapors formed during the cutting operation cover the protective caps with a comparatively thin metal coating, so that electric contact is made with the nozzle. In this manner, large, voltage-carrying areas may be produced on the burners which could be touched accidently during the operation.
These above mentioned protective devices must be regarded as dangerous because although they are called protective devices they can lose their protective function during the operation of the burner. Another more effective contact protective device is known which is free of the above mentioned drawbacks, but has technical disadvantages. In that device, the nozzle cap is separated from the nozzle by a ceramic insulator and a resilient seal. The protective cap is connected to the workpiece or earth potential via a fault current protective circuit. If the switching threshold of the protective circuit is exceeded by a suddenly occurring voltage potential, the plasma current source is disconnected. (DD PS No. 63 823, corresponding to U.S. Pat. No. 3,558,973, Pochert et al.)
This device has the drawback that the fault voltage monitoring system fails if the nozzle cap contacts the workpiece in difficult cutting positions, such as occur for example, in angular plates and elbows. If, in that case an arc flash-over or a metallic bridge formation takes place on the ceramic insulator, as a rule the insulator is destroyed by thermal overload. Another disadvantage is that the nozzle cap is exposed to electrolytic decomposition, caused by the high voltage potential between nozzle and nozzle cap. The cooling water acts in this case as an electrolyte. Due to this decomposition, the nozzle caps leaks at the places they are clamped.
It is the object of the invention, to avoid the drawbacks of the known devices and to provide a plasma burner which reliably meets the requirements of electrical contact protection.